Support is requested by a young investigator for a one-year pilot project to determine whether the absolute sensitivity of the human visual system changes from day to night in any predictable fashion. The psychophysical techniques are standard and familiar to the PI, and subjects can be recruited locally. The studies proposed include determing absolute rod and cone treshold at mid-day and mid-night in three groups of subjects: (1) persons who are normally active during the day, (2) persons who are normally active at night, and (3) persons who have been in darkness for 24 hours. The project is designed to provide some preliminary indication of whether the human visual system undergoes circadian changes similar to those recently demonstrated in animals. If so, this study will have both practical and clinical significance, for the knowledge that regular shifts in sensitivity occur daily in, for example, pilots, truck drivers, and patients should affect policy and diagnostic decisions.